Field
The present disclosure relates to techniques for communicating optical signals. More specifically, the present disclosure relates to a silicon optical modulator that includes a photonic crystal.
Related Art
Silicon photonics is a promising technology that can provide large communication bandwidth, large density, low latency and low power consumption for inter-chip and intra-chip connections. In the last few years, significant progress has been made in developing low-cost components for use in inter-chip and intra-chip silicon-photonic connections, including: high-bandwidth efficient silicon modulators, low-loss optical waveguides, wavelength-division-multiplexing (WDM) components, and high-speed CMOS optical-waveguide photo-detectors.
Electro-optical modulation in silicon can be implemented by exploiting the free-carrier dispersion effect of silicon, or by incorporating various electro-optical materials such as a polymer, germanium and III-V compound semiconductor into a silicon-on-insulator (SOI) platform. Because of CMOS compatibility, simplicity of fabrication and high operation speed, carrier-depletion-based modulation is typically used in optical modulators. In this approach, the concentration of free charges is used to change the real and imaginary parts of the index of refraction of silicon. In particular, under reverse bias, a pn junction embedded inside a silicon optical waveguide modulates its carrier-depletion region, therefore producing a phase shift of an optical signal. Note that this process can be very fast because it does not involve minority carrier diffusion.
It can be difficult to deplete large a amount of charge with a low driving voltage. The change in the index of refraction can be enhanced using a resonance effect (e.g., by using a micro-ring resonator). However, the extremely narrow bandwidth of most resonant devices usually limits their use over a wide range of temperatures. Alternatively, Mach-Zehnder interferometer (MZI) modulators have broad bandwidth that allows a wide temperature range of operation, but usually do not meet the requirements for optical interconnects because of their large size (usually in the millimeter range).
Recently, a slow-light structure with a low group velocity was implemented in MZI modulators to greatly increase the modulation efficiency, thereby reducing the device size dramatically. Note that the term ‘slow light’ refers to a reduction in the group velocity of light, and can be realized with many different approaches. One approach is based on material resonances, which usually requires bulky media, and low-temperature operation. Alternatively, another approach uses engineered photonic structures in which light bounces back and forth as it propagates forward, such as: a photonic crystal, a coupled resonator and/or waveguide-grating structures.
Photonic-crystal optical waveguides with flat-band slow light by band-gap engineering are usually chosen for the MZI modulator because they usually can be integrated with silicon optical waveguides, and because they provide a group index of refraction (ng) of more than 20, while still maintaining relatively large bandwidth (such as 20-30 nm). With such a photonic-crystal implementation, the phase-shifter length of the MZI modulator can be significantly reduced to the sub-100 μm range. This can eliminate the need for complex travelling-wave electrode designs in order to meet high-speed requirements. Instead, lumped devices with simple electrodes can be used in the MZI modulator for high-speed applications without increasing the applied voltage.
However, the optical waveguide loss also increases linearly with the ability to slow down the group velocity of light. In particular, even though the arm length of the MZI modulator with slow light can be reduced by an order-of-magnitude, the overall insertion loss of a slow-light MZI modulator is still typically similar to that of the normal designs. Moreover, the excessive mismatch in the group index of refraction between the channel optical waveguide and photonic-crystal optical waveguide introduces additional coupling loss. Furthermore, the operating bandwidth of the slow light is inversely related to the enhancement of the group index of refraction. Consequently, the slower the light travels, the narrower the operating bandwidth will be. These design tradeoffs often limit the shortest achievable MZI arm length, and eventually can set an upper limit on the RC-limited device bandwidth.
In principle, the modulation efficiency can also be greatly improved with novel pn junction designs. In particular, because the pn diode in an electro-optical modulator is formed inside the optical waveguide, the pn junction design (including the junction position, the junction profile and the doping level) can significantly affect the electro-optical modulator performance. However, it can be challenging to tailor the species and strengths of the doping to design a three-dimensional junction that maximizes the overlap with the optical mode, while also permitting standard, surface-normal fabrication.
Hence, what is needed is an electro-optical modulator without the above-described problems.